This invention relates generally to image processing and more particularly to a method and system for altering defects in a digital image.
Tangible images, such as photographic images, may have surface defects such as scratches, fingerprints, or dust particles. Such defects may occur, in the case of photographic images, in a transparency or negative as well as in a photographic print of a transparency or negative. Such defects often undesirably degrade a photographic image.
In the field of image processing, digital images derived from photographic images using a scanner most often include the defects present in the underlying photographic image. Because digital images are subject to mathematical manipulation, if image defects may be identified and distinguished from image detail, then those defects can be removed, either partially or completely.
A defect channel comprising a digital signal proportional to the defects in a photographic image may be created by scanning the photographic image using an infrared light source and an infrared light sensor. Infrared light will tend to pass through developed photographic film with nearly complete transmission because the dye in various layers of the photographic film does not fully absorb infrared light. On the other hand, where defects are present, a portion of the infrared light will tend to be refracted from the optical path before passing through the film. Thus, defects in the photographic image will tend to show up in a defect channel produced using an infrared light source and infrared sensor. In reflective scanners, a defect channel may be obtained by examining the difference between images obtained when the image being scanned is illuminated by light sources at different angles. The challenge is to use the defect channel to automatically alter defects in a digital image, while making as few undesirable changes to the digital image as possible.
One aspect of the invention is a method for altering defects in a digital image. At least a first pixel of a first channel of a digital image is filtered using digital circuitry to produce a filtered pixel by averaging the intensity of the first pixel and a plurality of additional pixels in the neighborhood of the first pixel. At least one of the pixels is weighted in response to the intensity value of at least one pixel in a defect channel associated with the digital image. A corrected digital image is produced using the digital circuitry in response to the filtered pixel and the first channel of the digital image.
The invention has several important technical advantages. Various embodiments of the invention may have none, one, some, or all of these advantages without departing from the scope of the invention. The invention allows automatic alteration of defects in a digital image based upon a defect channel having a signal proportional to defects in the digital image. The invention allows such alteration in the image substantially with little or no damage to the underlying image. Because the invention filters pixels of the digital image with pixels weighted based upon the expected reliability of each pixel, areas of strong defect are more easily excised without causing significant damage to the area surrounding the defect. Thus, for reasonably sized defects, the invention may automatically alter defects in a way that will most often produce a more pleasing image than the original image that included the defect. The desirability of the resulting image may depend upon the size of the defect and the amount of valid image information remaining in the digital image.